you said, that you recommenc a temperature difference of approximately 50°C between the refrigerants. Isn't the difference between R-600 and R-170 a bit high? According to a P/T-chart, ethane has a pressure of 27bar at 0°C
What about a mix of R-600/R-290/R-1150 or R-600/R-507a/R-1150? That would be closest to the "50°C-Rule"...
How much does Ethane in the US or in other countries than Austria cost? Here it is about 280€ for 3kg. A bit expensive